Hvac

[normanbk]

I have a 1999 Cadillac STS. It has 142,000 miles. In the past month, when the heat comes on it is fine for approx. 30 min. Then cold air starts to come out, not hot. This began after my repair shop filled my a/c with coolant and some kink of a sealer because it had a long standing leak. I took it back and was told that I had to manually turn off the A/C and the problem would go away. The day after I picked up the car, I followed the instructions, but the symtoms came back, again in approx. 30 miin. I googled the problem, and someone said that there is a software update that only GM can install and it will fix this problem. The cost is supposed to be $100.00. Is this correct and should I go to the GM dealer or is there something else wrong and some other way to fix the problem. I am located in Maryland and it has been very cold. I know from your show that the last part of my information is important, the care is Green